The waste generated by many industrial activities, as well as hazardous waste, is particularly problematic because of proper disposal requirements. Industrial waste containing heavy metals has to be treated before it is disposed. Left untreated, this waste could contaminate the environment and cause harm to life forms.
Several prior art processes have been proposed for treating and extracting toxic waste from industrial sludge. Many of these processes utilize various types of clay or shale to absorb or adsorb the heavy metals and other toxic materials from liquid and solid industrial waste.
Lo, I. M-C., et al., (1997, J. Envir. Engrg. Div., ASCE 123(1), pp 25-33) disclosed a method for the stabilisation of heavy metals in clay through heavy metal adsorption on the clay minerals. Adsorption can be external, occurring on the mineral surface, or internal, occurring within the mineral structure itself, for example, between the mineral layers of clay, as cations are attracted to the negatively charged clay particles.
Clay minerals are known to have a property termed Cation Exchange Capacity (LaGrega, M. D., 1994, McGraw-Hill, Inc., New York) whereby cations originally present in the clay structure are replaced by external cations. In fact, it is also through a cation exchange process that clay minerals, which consist of sheets of alumina and silica, acquire their overall negative charge. This happens when Si4+ cations are replaced by lower charged cations like Al3+, or when Al3+ cations are replaced by divalent cations like Fe2+, Mg2+ or Ca2+ (Weaver, C. E., 1989, “Clays, mud and shales.” Elsevier, New York). This negative charge makes it possible for heavy metals to be adsorbed on the clay mineral surfaces (external adsorption).
The metal stabilisation begins with the migration and adsorption of heavy metals on the mineral surfaces. Furthermore, layered clay minerals like montmorillonite are known to internally adsorb cations (Conner, J. R., 1990, Van Nostrand Reinhold, New York). Montmorillonite consists of individual layers, each made up of one alumina sheet between two silica sheets linked by cations like Ca2+. The interlayer cations can then be replaced by heavy metals through internal adsorption. Clay minerals have a basic alumina-silica structure and undergo chemical transformation when subjected to heat. These chemical transformations change the original chemical phases of the clay e.g. kaolinite and montmorillonite into mullite and cristobalite phases. The incorporation process of heavy metal cations is completed during the firing process through the formation of new phases in which the adsorbed heavy metals become incorporated. X-ray diffraction (XRD) analysis reveal that the firing process results in the transformation of kaolinite and montmorillonite, originally present in marine clay, into cristobalite and mullite, which have been reported to incorporate heavy metal cations (Schneider et al, 1994, Wiley and Sons, New York; Dion, L. B., 1996, Engineer Thesis, Stanford University, California, USA; Deer et al, 1992, 2nd edition, Longman Scientific & Technical, New York, Wiley).
The methods for clay stabilisation as described in the prior art can be generally distinguished as “encapsulation” and “incorporation” processes. Both processes involve a final step of vitrification of the matrix.
When heavy metals are present in the form of compounds or complexes, for example as metal oxides, the added clay-based ceramic minerals encapsule (or surround) the metal compounds. However, the metal compounds are not chemically bonded with the original clay-based ceramic minerals and are not transformed into a bonded crystalline phase. The encapsulation process can be carried out in the micro (<10 μm) or macro (>10 μm) state.
Clay-based ceramic matrix has a limited capacity and once its saturation capacity has been reached there will be no more incorporation and encapsulation taking place from there on. Once the sites of the matrix are filled, the clay has exhausted its stabilisation capability. Furthermore, different clay-based ceramic matrices have different incorporation and encapsulation abilities due to their varying chemical compositions. Further, encapsulation is not an efficient mechanism of metal stabilisation because once the surrounding clay matrix is disturbed or destroyed, the metals will be freed from the matrix and leach into the environment.
When the heavy metals are present in the form of heavy metal cations and metal compound anions, the heavy metal cations and metal compound anions chemically bind to and are incorporated into the clay. After firing at elevated temperatures, the heavy metals get incorporated into the mullite and cristobalite structure as part of the clay-based ceramic matrix. However, even if the heavy metals in this form are better stabilised than those stabilised with the encapsulation method, the stabilisation capacity for the clay is limited and the heavy metals need to be present as metal cations and anions before such incorporation could take place.
Clay has a limited stabilisation capacity based on the active adsorption sites within the clay matrix. However, when the incorporation capacity of heavy metals is exceeded, the heavy metals are simply encapsulated. Under this condition, when the clay matrix is disturbed, for example by external factors, heavy metals may leach out into the environment. Kaolinite, for example, is a layered structure with limited adsorption and ion exchange properties. Because it is a layered structure, the strength of the matrix is weak and can be destroyed by acid. Heavy metals may leach out as a result.
Accordingly, a substantial drawback of the above-described prior art is that the waste materials may not be environmentally safe in the long-run. These materials need to be kept under surveillance and are not suitable for uses like construction or land filling materials.
It would therefore be desirable to provide a process which would improve the stability of hazardous heavy metals and reduce or eliminate long-term storage risks while providing safe value-added products at the same time.